EGG SHAPED DIGESTERS WHAT REALLY MAKES THEM PERFORM

Egg Shaped Digesters (ESD) have been in North America since the early 1980's. Considerable experience has been gained with the existing and new ESD systems. Early systems mirrored the German experience with some American “improvements”. The question becomes; Have Egg Shaped Digesters performed as expected?American owners and engineers have made many design assumptions based on past American experience with flat pancakes, and the transfer of German technology. A number of facility design improvements and operation procedure changes have been developed in America to address “weaknesses” and to fully exploit the ESD strengths. The fundamental strengths of the ESD system, particularly the vessel shape, also create operational weaknesses when the necessary and unique design and operation procedures for ESD systems are not fully understood and carefully applied.During 2002 and 2003 all of the North American ESD facilities were visited and performance data obtained. Each site provided information about facility operation, as well as solids stabilization data. Several of the plants provided case histories and serve as design critique models. While German experience provides a long history for the development of ESD extending from 1927, American ESD facilities have proven that significant differences of operation, raw solids characteristics, and equipment application must be considered to optimize the application of ESD in North America.Several specific case histories of North American ESD facilities are presented in order to provide a comprehensive review of ESD application and performance while allowing the formulation of design and operation parameters and guidelines for the successful application of ESD design to new and expanding facilities. The specific case history facilities are:San Francisco, CA, Oceanside Water Pollution Control Plant.Midland, Ontario, Canada, Water TreatmentPlantBellaire, OH, Eastern Ohio Regional Wastewater Authority Facility.St. Charles, IL, Water Reclamation PlantLincoln, NE, Theresa Street Water Reclamation Plant.Hershey, PA, Derry Township Municipal Authority, Clearwater Road Wastewater Treatment FacilityIn addition to the detailed data and operation analysis for the six case history systems, a full database for all of the North American facilities is provided. The case histories provide the basis for design and operation recommendations to be applied to new and expansion facilities.Textbook theory indicates that thickened WAS does not stabilize well and does not produce significant quantities of gas. Actual ESD operation experience shows that the opposite occurs. TWAS when fed in slug doses can cause tremendous increase in process activity and gas production.One of the common problems encountered in the facilities in the case studies presented below is maintaining steady raw feed (volume and composition) and discharge rates in order to minimize sudden increases in gas production and attendant ancillary process problems that result. The operational surface area of an ESD system is much smaller than that of a comparable flat pancake digester. The zone below the liquid surface area is an inverted truncated cone. Minor changes in the concentration of entrained gas in the vessel can drive substantial changes in the liquid level because the volume change is reflected in substantial depth changes. When the biological activity substantially increases gas production, entrained gas content increases, thus contained liquid volume will try to increase. Under the stated conditions and because the ESD is a constant volume vessel system, the discharge system operates at a higher output level than the incoming raw solids flow. When the slug of food is utilized and the gas production decreases, the outflow stops, gas escapes and the apparent volume decreases because of lost entrained gas volume. The liquid level falls abruptly and for no apparent reason.Based on the experience of the existing North American ESD facilities, design and operation procedures are developed and explained.